XS is an obfuscated programming language, devised by Mark Carter, which is based on XML.
XS has two commands: <print> and a for loop.
<print> does the obvious: prints what is between the <print> tag and </print> tag.
The for loop uses the <for> tag, which has the attributes "var" "from" "to" and "step". "var" is rather superfluous as this is never used again. From is the initial value of var and to is the ending value. Step is the increment of var.
Note: XS is possibly a joke.
Also note: There is some dispute as to whether XML is possibly a joke.